Electronic circuit assemblies are often required to be capable of surviving in hostile service environments, including those commonly found in automotive and aerospace applications. Such assemblies often employ flip chip integrated circuits (IC), which are generally characterized as being electrically and mechanically attached to an electronic circuit assembly with a number of solder joints. The solder joints are typically formed by solder bumps that were registered with conductor traces on the substrate surface of the assembly, and then reflowed to bond the flip chip to the traces. The solder joints are subject to thermal stresses as a result of temperature fluctuations in the service environment of the assembly, and differences in coefficients of thermal expansion of the various materials that form the flip chip, substrate and solder joints, which are typically silicon, alumina and a tin-lead alloy, respectively. These thermal stresses can potentially fatigue and fracture the solder joints, particularly if the assembly is subject to many temperature excursions, high temperatures on the order of 125.degree. C. or more, or intense vibration. Under such conditions, the expected life of the solder joints can be significantly decreased.
The above is aggravated as the size of the flip chip increases, which produces larger dimensional changes relative to a given temperature change. Even greater mismatches arise from current technology in which organic materials are used to form the substrates to which flip chips are mounted organic materials such as epoxy/glass, polyimide/glass and epoxy/aramid, tend to have a larger coefficient of thermal expansion mismatch with the silicon material of flip chips as compared to traditional ceramic substrate materials, and therefore their use tends to encourage thermal cycle fatigue fractures of the solder joints.
Improved solder joint life has been achieved by encapsulating solder joints with an encapsulation material having a coefficient of thermal expansion approximately equal to that of the solder joint material, e.g., within about 20 percent of the coefficient of thermal expansion of the solder joint to be encapsulated. Because the solder joints of a flip chip are beneath the chip and support the chip above the surface of the substrate, the encapsulation material must have adequate pre-cure flow characteristics to enable the material to be drawn by capillary forces beneath the chip and between adjacent solder joints to completely encapsulate each solder joint. Suitable encapsulation materials for this purpose have been formed by epoxies in which a glass filler material is dispersed in order to reduce the coefficient of thermal expansion of the material to a value closer to that of the solder joint material. Because gaps on the order of about 0.1 millimeter (about 4 mils) or less are typical between flip chips and their substrates, there is a tendency for non-uniform flow of the encapsulation material to trap air in pockets beneath the flip chip. If an air pocket forms a void at or near a solder joint, the thermal cycle fatigue life of the solder joint will likely be adversely affected. However, the location of the solder joints beneath the flip chip and the small gap between the flip chip and its substrate make visual inspection impossible.
Accordingly, it would be desirable if an inspection technique were available that could detect air pockets and voids in an encapsulation material used to encapsulate the solder joints of an IC device. Such a technique would preferably be amenable to in-line processing methods, and entail an encapsulation material that facilitates the inspection technique yet is compatible with the preferred flip chip, solder joint and substrate materials, particularly in terms of coefficient of thermal expansion, even if the substrate material is an organic laminate.